HP 25 low battery indication at 2.6 v

[Peter Klein]

My Panamatik-enhanced HP 25 has a small annoyance. I run it on Eneloop rechargeable batteries. The low battery indicator (multiple decimal points) comes on at slightly over 2.6v. It also did this before I installed Bernard’s ACT board (standard, not Low Power). So the problem must be somewhere other than the ACT. Is there a component I could replace that would fix this? Is it worth doing?

This must be a known issue, because Bernhard included a setting that will disable the low battery indicator. With this done, my calculator works correctly down to 2.2v (measured with no load). Then the display starts to flicker as if a program is running. Nothing has been lost or scrambled when this happens. So far, anyway.

—Peter

[teenix]

The low voltage detect is part of the cathode driver IC. There is a small potentiometer where you can adjust the trip point.

See attached

cheers

Tony

[Peter Klein]

Thanks, Teenix. I'll tweak that pot the next time I go inside the calculator to re-clean a couple of stubborn dirty keys.

I love hpmuseum. No matter what you want to know, there's somebody here who knows it.

--Peter

[Peter Klein]

No joy, unfortunately. I hooked the HP-25 up to an adjustable bench power supply, whose accuracy I verified with a DVM. I adjusted the pot to a number of positions along its arc. Once I began adjusting it, the low battery indicator stayed on at all voltages between 3.3 and 1.8.v. Lower than 1.8v and it shuts off. I cycled power on off and on after each adjustment.

In each case, the result was the same: Power on. Calculator displayed opening message. I keyed in the digit “1” and pressed Enter. The low battery indicator came on. This was unaffected by the number of digits displayed.

I also reflowed the solder points of the pot and nearby pins of the cathode driver. And I checked for solder bridges with a magnifier. No help. Current drain appears to be about right. I suspect the pot has failed internally.

So I’ve turned the low battery indicator off using Bernhard’s flag. Unless someone has other ideas, I guess I’ll rely on the operating time function and/or periodic battery checks.

[teenix]

Apologies I forgot you are not running the original hardware.

The original cathode driver IC was responsible for the low battery indication. As far as I know, this is removed when using Bernhards cool kit. If so, the low battery indication must be set somehow from the new hardware.

cheers

Tony

[PANAMATIK]

(01-07-2024 10:35 AM)teenix Wrote:  Apologies I forgot you are not running the original hardware.

The original cathode driver IC was responsible for the low battery indication. As far as I know, this is removed when using Bernhards cool kit. If so, the low battery indication must be set somehow from the new hardware.

cheers

Tony

The battery low indication is done via digital input pin 3 of the ACT chip, the same in original and new ACT. There is a comparator built in the cathode driver 1818-0153, which compares the current battery voltage with the potentiometer voltage and generates a digital signal at output pin 17. This signal is connected to pin 3 of the ACT. The dots will be shown by software then and will be updated only when you press a button to avoid flicker at the treshold voltage. The new ACT uses the original HP code to display the low battery dots.

Bernhard

[teenix]

Oops, poor choice of words on my part, I probably should have said the Cathode Driver is responsible for detecting the low battery state.

cheers

Tony